In general, combustion apparatuses such as boilers and water heaters for the purpose of use of heating and hot water are classified into an oil boiler, a gas boiler, an electric boiler and a water heater depending on the fuel it is supplied with, and are variously developed and used according to installation applications.
In such combustion apparatuses, particularly the gas boiler and the water heater typically use a bunsen burner or a premixed burner to burn gas fuel. The premixed burner among others has a combustion method of mixing gas and air in a mixing ratio for optimal combustion and then supplying a mixture (air+gas) to a flame hole section so that the mixture is burned.
In addition, the performance of the combustion apparatuses is evaluated as a TDR (Turn-Down Ratio). The TDR refers to “a ratio of maximum gas consumption to minimum gas consumption” in a gas combustion device in which an amount of gas is variably controlled. For example, when the maximum gas consumption is 24,000 kcal/h and the minimum gas consumption is 8,000 kcal/h, the TDR is 3:1. The TDR is constrained by whether flame is stably maintained to some degree under minimum gas consumption.
In the gas boiler and the water heater, convenience for use of heating and hot water is increased as the TDR becomes greater. That is, when the burner is actuated in a region in which the TDR is small (namely, the minimum gas consumption is high) and loads of heating and hot water are small, the combustion apparatuses are frequently turned on/off. For this reason, variation in temperature control is increased and durability of the apparatuses is deteriorated. Thus, in order to improve these problems, various methods for improving the TDR of the burner applied to the combustion apparatuses have been developed.
In such a modulating control burner, valves allowing for supply of gas are mainly classified into an electrical modulating gas valve controlled by current values and a pneumatic modulating gas valve controlled by differential pressures generated during supply of air.
The pneumatic modulating gas valve controls an amount of gas supplied to the burner by differential pressures generated when air required for combustion is supplied to the burner by a blower. In this case, air and gas required for combustion are mixed in a gas-air mixer and then supplied to the burner in a mixture (air+gas) form.
In the gas-air mixer of the gas burner using the above pneumatic modulating gas valve, the TDR is basically constrained by a factor such as a relation between gas consumption Q and differential pressure ΔP. The relation between flow rate and differential pressure of a fluid is generally as follows.Q=k√{square root over (ΔP)}
That is, as seen in the above relational equation, in order to double the flow rate of the fluid, the differential pressure has to be quadrupled.
Accordingly, the ratio of pressure differences has to be defined as 9:1 for defining the TDR as 3:1 and the ratio of pressure differences has to be defined as 100:1 for defining the TDR as 10:1. However, there is a problem in that it is impossible to infinitely increase gas supply pressure.
In order to resolve the problem in which the gas supply pressure may not be infinitely increased, there is disclosed a method which increases the TDR of the gas burner by respectively dividing passages, through which air and gas are supplied, into two or more regions and by opening and closing each passage of gas injected into the burner.